(CNN) -- Greenpeace activists who have interfered with the launch of a Chevron oil drilling ship in the North Sea say that they have enough fuel and supplies to last several weeks.

Activists on board the Greenpeace ship Esperanza attached a bright yellow "survival pod" to the giant ship's anchor on Wednesday, effectively keeping it from going to sea.

The drill ship Stena Carron, operated under contract by oil giant Chevron, is now holding off the Shetland Islands in the North Sea.

Two members of the crew shimmied up the chains, spending Tuesday night hanging above the waves in a small tent. They then secured the survival pod, which is now occupied by two new activists.

Inside the ball, there's enough food, fuel and water and even a small toilet to sustain the occupants for up to a month, Greenpeace says.

Ben Stewart, leader of the operation, told CNN via satellite phone from the scene that a storm is approaching, but the crew and the activists are safe.

The bright yellow "survival pod" attached to the giant ship's anchor effectively keeps it from going to sea. The 2-meter diameter pod is hanging from the chain, preventing it from being raised high enough for the 748-foot (228-meter) ship to safely embark.

Stewart says that they will stay as long as possible, adding "every day they are held back is a day that we can avoid the chance of another Gulf of Mexico-style environmental disaster in the North Sea."

The group hopes to pressure Chevron and European Union environmental ministers gathering this week in Norway to support a deepwater drilling moratorium in the North Sea.

Chevron says it has been in contact with the activists and stressed the need for safety.

"We fully acknowledge and respect the right of Greenpeace or anyone else to express their views by peaceful and lawful action." Chevron said in a press release. "However, we are deeply concerned about activities that put people at risk."

Greenpeace says it has assured the captain of the Stena Carron it will not interfere with the second anchor so the ship will not float into danger.

It's the second such boarding action by the Greenpeace team in recent weeks. They boarded an oil drilling rig operated by British company Cairn Energy in Arctic waters off Greenland in August.